The exemplary embodiments and/or exemplary methods of the present invention relate to a steering device for a vehicle.
A steering device for a vehicle is discussed in German patent document no. 198 03 745. The vehicle discussed there has a freely steerable axle which is also referred to as a self steering axle and whose steering can be locked as a function of predefined operating states of the vehicle. In particular, the intention is that this steering system will be locked when a minimum velocity of the vehicle is exceeded. Furthermore, sensors for sensing the velocity of the vehicle, the steering angle of the steerable axle and brake pressure are provided and are connected to an electronic control device. Depending on predefined measured values of the sensors, the electronic control device releases a locking device for the self steering axle or activates it.
German patent document no. 100 65 186 discusses a hydraulically activated steering device of a utility vehicle with two axles which can be steered actively by a hydraulic system. An electronically controllable damping valve is connected into the hydraulic circuit and can be adjusted to a stored characteristic curve as a function of the load state of the vehicle.
German patent document no. 198 12 238 discusses a method for controlling the yaw behavior of vehicles. Sensors for the vehicle velocity and the steering angles are connected to a control unit which determines the yaw rate of the vehicle. Two independent control circuits for a steering intervention and a braking intervention control the yaw rate in accordance with a predefined setpoint value.
Heavy trucks and many buses have a third axle in addition to a front axle which can be steered by the driver and a driven, nonsteerable rear axle. If the third axle is arranged behind the driven axle, it is referred to as a “trailing axle”.
A distinction is made between vehicles in which the third axle is rigid, i.e. cannot be steered, and the vehicles in which it is steerable. In the case of steerable third axles, a distinction is further made between axles which are coupled kinematically to the front axle steering system, i.e. which are also steered automatically by the driver together with the front axle and what are referred to as “lateral force steered axles”. Lateral force steered axles are not coupled to the steering system of the vehicle and are therefore also referred to as “freely steerable axles”. If the vehicle travels round a bend, laterally directed positive forces arise between the underlying surface and the wheels of the lateral force steered axle and lead to an “automatic” steering lock.
In comparison to other multiaxle vehicles in which a plurality of axles are arranged rigidly, i.e. one behind the other in a nonsteerable fashion, the wear on the tires in vehicles with steerable “additional axles” is less, in particular when cornering at low velocities, and the vehicle has a better turning circle. However it is considered to be disadvantageous that vehicles with one or more freely steerable axles have a smaller degree of “lateral rigidity” and thus worse stability on bends compared to vehicles with nonsteerable axles.
In particular in the case of low coefficients of friction and on a smooth underlying surface, vehicles with freely steerable axles tend to oversteer more readily than vehicles with rigid axles. The tendency to oversteer is increased further if the nonsteered axle is driven and also has to transmit drive forces in addition to the lateral guiding forces.
As understood, vehicles with a second rear axle are known which are steered by lateral force and in which the lateral force steered axle can be locked “where necessary”, i.e. in which the degree of freedom of the steering can be locked. The locking or blocking is carried out here exclusively as a function of the velocity of the vehicle, with the axle being locked above a specific minimum velocity. Such vehicles have significantly improved lateral rigidity at relatively high velocities compared to vehicles with a lateral force steered axle which cannot be locked. At the same time, the free steerablility at low velocities allows the wear on tires to be reduced.